Thermal interface solution with reduced adhesion force

ABSTRACT

A method comprises applying an adhesive to a first substrate and a second substrate to secure the first substrate to the second substrate. The adhesive extends in a plane on one side of an interposer that also extends in the plane, and is contiguous with the adhesive. The interposer comprises openings to enable flow of adhesive through the openings to form adhesive bond areas on one of the substrates where the areas substantially conform to the openings and lie adjacent to adhesive free areas. The adhesive substantially covers the other of the substrates so that the bond areas produce regions of reduced adhesive strength to the one substrate compared to the bond strength of the adhesive to the other substrate. Adjusting opening sizes adjusts area bond strengths. One substrate may comprise a VTM, the other a heat spreader, and the adhesive, a TIM. An article of manufacture comprises the substrate-adhesive-interposer-adhesive-substrate layers.

This application is a Continuation Application pursuant to 35 U.S.C. §120 of Parent Application Ser. No. 14/299,547 filed: Jun. 9, 2014 whichis incorporated herein by reference in its entirety, the presentapplication also claiming the benefits of this Parent Applicationpursuant to 35 U.S.C. § 120.

FIELD OF THE INVENTION

The field of the invention in one aspect comprises a thermal interfacestructure made up of a thermal interface material (“TIM”) on aninterposer that controls the area of contact of the TIM to a substrate.

BACKGROUND OF THE INVENTION

The so-called “silicon revolution” brought about the development offaster and larger computers beginning in the early 1960's withpredictions of rapid growth because of the increasing numbers oftransistors packed into integrated circuits with estimates they woulddouble every two years. Since 1975, however, they doubled about every 18months.

An active period of innovation in the 1970's followed in the areas ofcircuit design, chip architecture, design aids, processes, tools,testing, manufacturing architecture, and manufacturing discipline. Thecombination of these disciplines brought about the VLSI era and theability to mass-produce chips with 100,000 transistors per chip at theend of the 1980's, succeeding the large scale Integration (“LSI”) era ofthe 1970's with only 1,000 transistors per chip. (Carre, H. et al.“Semiconductor Manufacturing Technology at IBM”, IBM J. RES. DEVELOP.,VOL 26, no. 5, September 1982). Mescia et al. also describe theindustrial scale manufacture of these VLSI devices. (Mescia, N. C. etal. “Plant Automation in a Structured Distributed System Environment,”IBM J. RES. DEVELOP., VOL 26, no. 4, July 1982).

The release of IBM's Power6™ chip in 2007, noted “miniaturization hasallowed chipmakers to make chips faster by cramming more transistors ona single slice of silicon, to the point where high-end processors havehundreds of millions of transistors. But the process also tends to makechips run hotter, and engineers have been trying to figure out how tokeep shrinking chips down while avoiding them frying their owncircuitry.”(http://www.nytimes.com/reuters/technology/tech-ibm-power.html?pagewanted=print(2/7/2006))

Technology scaling of semiconductor devices to 90 nm and below hasprovided many benefits in the field of microelectronics, but hasintroduced new considerations as well. While smaller chip geometriesresult in higher levels of on-chip integration and performance, highercurrent and power densities, increased leakage currents, and low-kdielectrics with poorer heat conductivity occur that present newchallenges to package and heat dissipation designs.

CMOS power density is increasing. Recently the industry has seen it risefrom 100 W/sq cm to 200 W/sq cm, beyond that of bipolar technology inthe early 1990's. This increase in power density also increases theoperating temperature of the device which materially interfered withproper operation of the device. The industry addressed this increase inoperating temperature by securing the device to a heat exchangestructure or material (i.e., heat spreader), but different coefficientsof expansion of the heat spreader and the device caused structural andconsequently further operating problems in the device. The difficultywas resolved for the most part by placing a TIM between the two that notonly joined them in a heat exchange relation but also providedsufficient flexibility that enabled a link between the surfaces thatsubstantially compensated for their different coefficients of expansionsand substantially minimized any stress or strain placed on the device inthe heat exchange process. The TIM material typically would situnderneath “contact patches,” helping to thermally bridge the gapbetween the device being cooled and the “contact patch.”

New generation servers employ more vendor technology that usuallyrequires some level of custom integration to realize reliableperformance. “VTM” (voltage transmission module) is one such technologyused for power control in servers developed today. Kim, et al., U.S.Pat. Nos. 8,498,540 and 8,565,606 describe circuits used in voltagetransmission module technology. The VTM requires a TIM to effectivelytransfer heat from an array of VTMs to a common heat spreader. Inherentin manufacturing a VTM printed circuit board (PCB) assembly is rework.Thus, the common heat spreader must be easily separable from the VTMarray in order to remove and replace a failed module. The heat spreaderremoval process must not damage any good modules.

The VTM has fragile solder connections that can only tolerate acompressive limit of about 15 psi and about 7 psi of tensile stress.TIMs that separate at tensile stress less than 7 psi either lackcompliance to accommodate the bondline tolerances as in the case ofthermal pads; or, in the case of accommodating bondline tolerances, forexample greases or low cross link density gels, lack positionalstability and pump out due to thermal mechanical movement between thecommon heat spreader and large PCB with the array of modules. Curablepaste, adhesive TIMs that accommodate bondline tolerances and which aremechanically stable will require tensile forces greater than 7 psi toseparate and thus will damage good components.

The application requirements for thermal interface materials thereforecan include both assembly (compressive) and disassembly (tensile) forcerequirements. As noted above, the VTM module has a compressive limit ofabout 15 psi and tensile limit of about 7 psi. These requirements havelimited the selection of thermal interface materials in the past to onlyone candidate, Chomerics T636, however, the present invention allows theuse of many other commercial TIM's as well as newly formulated TIM's.

The present invention proposes a method for controlling TIM force fordisassembly which would meet this and all other applicationrequirements.

RELATED ART

Schuette et al., U.S. Pat. No. 7,738,252; Furman, et al., U.S. Pat. No.7,694,719; Gruendler et al., U.S. Pat. No. 7,688,592; Thompson et al.,U.S. Pat. No. 7,646,608; Mok et al., U.S. Pat. No. 7,002,247; Solbrekkenet al., U.S. Pat. No. 6,523,608; Edwards et al., U.S. Pat. No.6,275,381; Lee et al. U.S. Pat. No. 6,050,832; Deeney, U.S. Pat. No.5,783,862; Rhoades et al., U.S. Pat. No. 4,151,547; Hill et al. UnitedStates Patent Application Publication 2010/0321895; Pang TIM SelectionCriteria for Silicon Validation Environment; 26 th IEEE SEMI-THERMSymposium 2010, pp.107 et seq. all show heat transfer devices fordissipating heat from a heat generating body.

SUMMARY OF THE INVENTION

The present invention provides structures, articles of manufacture andprocesses that address these needs to not only provide advantages overthe related art, but also to substantially obviate one or more of theforegoing and other limitations and disadvantages of the related art byproviding a thermal interface structure made up of a TIM on aninterposer that controls the area of the contact surface of the TIM on asurface from which or to which heat is to be transferred.

Not only do the written description, claims, and abstract of thedisclosure set forth various features, objectives, and advantages of theinvention and how they may be realized and obtained, but these features,objectives, and advantages will also become apparent by practicing theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not necessarily drawn to scale butnonetheless set out the invention, and are included to illustratevarious embodiments of the invention, and together with thisspecification also serve to explain the principles of the invention.These drawings comprise various Figures that illustrate, inter aliastructures and methods for adjusting the bonding strength of adhesivessuch as TIMs to substrates.

FIG. 1 and FIG. 1A illustrate an aspect of the present inventioncomprising a side elevation in cross-section of a perforated interposercoated with an adhesive on one side where the adhesive can extendthrough the openings toward the other side of the interposer.

FIG. 2 Illustrates an aspect of the present invention comprising a planview of a perforated interposer.

FIG. 3 illustrates an aspect of the present invention comprising a sideelevation in cross-section of a perforated interposer coated with anadhesive on one side, the adhesive also extending through theperforations toward the other side of the interposer. Substrates areplaced on either side of the interposer where the adhesive on one sideof the interposer substantially contacts the entire face of one of thesubstrates, and the adhesive projecting through the perforationscontacts limited areas on the other of the substrates.

DETAILED DESCRIPTION OF THE INVENTION

To achieve the foregoing and other advantages, and in accordance withthe purpose of this invention as embodied and broadly described herein,the following detailed description comprises disclosed examples of theinvention that can be embodied in various forms.

The specific processes, compounds, compositions, and structural detailsset out herein not only comprise a basis for the claims and a basis forteaching one skilled in the art how to employ the present invention inany novel and useful way, but also provide a description of how to makeand use this invention. The written description, claims, abstract of thedisclosure, and the drawings that follow set forth various features,objectives, and advantages of the invention and how they may be realizedand obtained. These features, objectives, and advantages will alsobecome apparent by practicing the invention.

The invention comprises, inter alia, a process for reducing the area ofcontact between a thermal interface material and a module which is to becooled. The disassembly force of a thermal adhesive is equal to theadhesive force per unit area (e.g. pounds/square inch) times the totalarea of contact (square inches). This invention allows control over thedisassembly force to protect fragile components from damage due toseparation forces. This invention provides the ability to accommodatelarge bondline tolerances on the order of about +/−0.5 mm or more andwhich are mechanically stable when using curable paste adhesive TIMshaving reduced bond area.

Reducing the bond area is accomplished by using an adhesive such as aTIM known in the art, and in one embodiment, a curable or cured TIM, incombination with an interposer sheet or film, e.g., a thermallyconductive interposer sheet. The interposer has holes or perforationsthat allow a layer of the thermal paste adhesive positioned on and aboveone face of the interposer to continuously bond to a heat spreader abovethe interposer both in areas above the perforations and in areasadjacent the perforations. The layer of the thermal paste adhesive isalso made to extend through the perforations to the surface of acomponent (e.g., PCB) beneath the interposer to bond the heat spreaderto the component as well. However, in the web area (non-perforated area)of the interposer, the thermal paste adhesive is blocked from contactinga portion of the component beneath the interposer to which it wouldnormally bond in the absence of the interposer.

By reducing the bond area of the curable TIM below the interposer, thetensile stress to separate the heat spreader from the array ofcomponents on the PCB is lowered and controlled to below the fragilitylimit of the component. We therefore enable a low thermal resistance ofthe interposer contacting a surface by coating the interposer with theTIM, and in one embodiment an elastomeric TIM that is fully cured orcurable in situ and has low or no tack. We illustrate this TIM aselement 16 in FIG. 1.

Although the interposer side with the TIM coating may be placed in drycontact with either the electrical device component or heat spreadersurface, in the Figures, we show the interposer side with the TIMcoating placed in dry contact with the heat spreader surface. Becausethe TIM coating is soft and compliant, the mating force used to join theheat spreader with the components will promote good thermal contactbetween the TIM coating and the surface to which it is mated. FIGS. 1and 2 show a thin (0.003 inches) interposer, e.g., copper foil (12, 22respectively) coated with a 0.003 inch thick layer of high performancecurable TIM (16) that is preapplied to a first side of the interposereither as a curable paste and cured in place or, already cured thermalpolymer pad with a tacky surface for adhering to the interposer.

The TIM coated interposer 12, 22 is cut to the size of a typical VTM,about 22 to about 32.5 mm. Five holes, (14, 24), about 9 mm in diameter,are punched in the interposer 12, 22, i.e., we coat the 0.003 inch thickcopper foil interposer (12, 22) with a high performance curable pasteTIM and then add the five, 9 mm diameter holes. The surface of theinterposer foil (12, 22) that is coated with TIM 16 is placed in directcontact with a VTM substrate surface. The preapplied TIM, 16, is fullycured and has low adhesion, that can range from substantially noadhesion to less than two psi in tension. As shown in FIG. 1A, we thendispense, as normal, a curable paste TIM thermal adhesive, 17 on thesecond side of the copper foil interposer (12, 22).

In another embodiment, we coat an interposer or metal foil (12, 22) witha TIM (16). The TIM can be either a thermal pad that has an adhesive onone side that allows it to stick to the interposer; or, it can be anadhesive paste TIM that can be applied by stencil and screen printingand then cured. In case, (thermal pad or curable paste), the resultingstructure is the interposer (metal foil) with a continuous coating ofTIM. Next, holes are punched in the two layer structure. The total areaof the holes is the area over which a second curable TIM, 17, will bond.After the holes are punched, the two layer structure is placed on acomponent for example the VTM. The TIM layer, 16, is placed on thecomponent. TIM layer, 16, has very low adhesion to the VTM because it isalready completely cured. But, it is allowed that TIM layer, 16, canhave some tack, enough so that it does not move when placed on the VTM.Next, as shown in FIG. 1A, the second TIM 17, a paste and curablematerial is dispensed over the two layer structure that is in place onthe VTM.

The second TIM, 17, only contacts the VTM in the area where there areperforations or holes in the two layer structure and this is thecontrolled bond area that will, by design, allow tensile separationforces less that 7 psi being applied over the entire area of the VTM.After the second TIM, 17, is dispensed, the heat spreader is mated. Thesecond TIM, 17, is a paste and therefore can accommodate a large rangeof bond lines with large tolerances that result from a PCB with manyvarieties of components that are interfaced to a common heat spreader.

Lastly, the heat spreader, (38 in FIG. 3) is mated with force over aperiod of time to the VTM substrate surface below. In this example,adhesion is developed to the VTM substrate surface over only 44% of itsarea (area of holes or perforations 14, 24) and the correspondingtensile stresses to separate the heat spreader are reduced by 56% asthis is the area that is blocked by the interposer 12, 22. The reductionof the area of adhesive to one of the substrates compared to the area ofthe adhesive on the other substrate is substantially proportional to thereduction of adhesive strength to the substrate with the lesser amountof adhesive.

FIG. 3 comprises an illustration of substrates 38and 40 bonded to aninterposer 32 having perforations 34. A high performance curable pasteTIM thermal adhesive 36 extends over the top of interposer 32 toadhesively bond interposer 32 to substrate 38. Adhesive 36 also extendsdownward through perforations 34 and adhesively bonds to substrate 40 inadhesive areas on substrate 40 that substantiality conform to thedimensions of perforations 34 where these adhesive areas are adjacent tosubstantially adhesive free areas on substrate 40. Either one ofsubstrates 38and 40 may comprise a heat spreader and the other anelectrical device such as a PCB, e.g., a VTM.

Interposer 12, 22, and 32 comprises a foil from about 0.001 inch toabout 0.005 inches in thickness and can be made of any material known inthe art, but especially heat conductive materials such as metals, e.g.,copper, silver, gold, aluminum, titanium, tungsten and the like andalloys thereof, or carbon fibers or carbon nanotube sheets orcombinations thereof, including multilayer devices that include anycombination of these materials.

In one embodiment, illustrated in FIG. 3, we provide an article ofmanufacture comprising an adhesive 36 on a first substrate 38 and asecond substrate 40. The adhesive 36 secures the first substrate to thesecond substrate 40, where the adhesive 36 extends in a plane on oneside of an interposer 32. The interposer also extends in the plane andis contiguous with the adhesive 36. The interposer comprises openings orperforations 34 through which the adhesive 36 flows to define adhesivebond areas on the second substrate 40. The bond areas substantiallyconform to the openings or perforations and are adjacent to adhesivefree areas. The first substrate 38 is substantially covered by theadhesive 36, so that the bond areas comprise regions of reduced adhesivestrength to the second substrate 40 compared to the strength of the bondof the adhesive 36 to the first substrate 38. In other embodiments, oneof the substrates may comprise a heatable substrate and the other of thesubstrates comprises a heat spreader substrate. The interposer maycomprise a heat conductive material. One of the substrates may alsocomprise an electronic device such as a printed circuit board.

The adhesive includes high performance curable paste or cured TIMthermal adhesives known in the art or any other TIM material, such asgreases, gels, phase change materials and the like, all of which aredescribed by A. Gowda, et al., Solid State Technology, “Choosing theRight Thermal Interface Material,” Volume 14, Issue 3, 2005. The presentinvention allows for the selection of a TIM adhesive from the manycommercially available TIM adhesives since the bond strength of theseadhesives to fragile substrates is now adjustable by employing theinterposer of the invention.

A second embodiment is to use a thin thermal pad with low to no tack andplace it on the VTM surface. Next, we dispense the second TIM, 17, ontop of the thermal pad and then mate the heat spreader. The very lowadhesion of the thin thermal pad will allow easy separation. The use ofthe second TIM, 17, a curable paste adhesive, accommodates large rangesand tolerances in bond lines that cannot be accommodated by thermalpads. While this is a much simpler solution, the benefit of the firstembodiment is that by personalizing the perforations in the two layerstructure, these can be located over hot spot areas and therefore a veryhigh performance TIM can be used in the these areas and have directcontact to hot spots.

Various embodiments of the invention also comprise inter alia a methodcomprising applying an adhesive to a first substrate and a secondsubstrate to secure the first substrate to the second substrate, theadhesive extending in a plane on one side of an interposer, theinterposer also extending in the plane and contiguous with the adhesive,the interposer comprising openings to enable the flow of the adhesivethrough the openings to form adhesive bond areas on one of thesubstrates that substantially conform to the openings, the bond areasbeing adjacent to adhesive free areas, the other of the substrate beingsubstantially covered by the adhesive, so that the adhesive bond areasproduce regions of reduced adhesive strength to the one substratecompared to the strength of the bond of the adhesive to the othersubstrate, where one of the substrates comprises a VTM electronicdevice, and the other of the substrates comprises a heat spreadersubstrate. The VTM electronic device may comprise a heatable substrateand the other of the substrates comprises a heat spreader substrate; theinterposer, a heat conductive material; and the adhesive a TIM. Theopenings may be sized to produce the bond areas that allow separation ofthe heat spreader from the VTM without damaging the VTM, and may besized to produce the bond areas that allow separation of the heatspreader from the VTM without damaging the VTM, where the VTM has acompressive limit of about 15 psi and tensile limit of about 7 psi. In afurther embodiment the TIM may comprise a curable or a cured TIM.

In a further embodiment the invention also comprises inter alia anarticle of manufacture comprising an adhesive on a first substrate and asecond substrate, the adhesive securing the first substrate to thesecond substrate, the adhesive extending in a plane on one side of aninterposer, the interposer also extending in the plane and contiguouswith the adhesive, the interposer comprising openings through which theadhesive flows and defines adhesive bond areas on one of the substrates,the bond areas substantially conforming to the openings, the bond areasbeing adjacent to adhesive free areas, the other of the substrate beingsubstantially covered by the adhesive, so that the bond areas compriseregions of reduced adhesive strength to the one substrate compared tothe strength of the bond of the adhesive to the other substrate, whereone of the substrates comprises a VTM electronic device, and the otherof the substrates comprises a heat spreader substrate. The VIMelectronic device may comprises a heatable substrate and the other ofthe substrates comprises a heat spreader substrate; the interposercomprises a heat conductive material; the adhesive comprises a TIM; theopenings may be sized to produce the bond areas that allow separation ofthe heat spreader from the VTM electronic device without damaging theVTM; the openings may be sized to produce the bond areas that allowseparation of the heat spreader from the VTM without damaging the VTM,where the VTM has a compressive limit of about 15 psi and tensile limitof about 7 psi; and the TIM comprises a curable or a cured TIM.

Throughout this specification, and abstract of the disclosure, theinventors have set out equivalents, of various materials as well ascombinations of elements, materials, compounds, compositions,conditions, processes, structures and the like, and even though set outindividually, also include combinations of these equivalents such as thetwo component, three component, or four component combinations, or moreas well as combinations of such equivalent elements, materials,compositions conditions, processes, structures and the like in anyratios or in any manner.

Additionally, the various numerical ranges describing the invention asset forth throughout the specification also includes any combination ofthe lower ends of the ranges with the higher ends of the ranges, and anysingle numerical value, or any single numerical value that will reducethe scope of the lower limits of the range or the scope of the higherlimits of the range, and also includes ranges falling within any ofthese ranges.

The terms “about,” “substantial,” or “substantially” as applied to anyclaim or any parameters herein, such as a numerical value, includingvalues used to describe numerical ranges, means slight variations in theparameter or the meaning ordinarily ascribed to these terms by a personwith ordinary skill in the art. In another embodiment, the terms“about,” “substantial,” or “substantially,” when employed to definenumerical parameter include, e.g., a variation up to five per-cent, tenper-cent, or 15 per-cent, or somewhat higher. Applicants intend thatterms used in the as filed or amended written description and claims ofthis application that are in the plural or singular shall also beconstrued to include both the singular and plural respectively whenconstruing the scope of the present invention.

All scientific journal articles and other articles, including internetsites, as well as issued and pending patents that this writtendescription or applicants' Invention Disclosure Statements mention,including the references cited in such scientific journal articles andother articles, including internet sites, and such patents, areincorporated herein by reference in their entirety and for the purposecited in this written description and for all other disclosurescontained in such scientific journal articles and other articles,including Internet sites as well as patents and the references citedtherein, as all or any one may bear on or apply in whole or in part, notonly to the foregoing written description, but also the followingclaims, and abstract of the disclosure.

Although we describe the invention by reference to some embodiments,other embodiments defined by the doctrine of equivalents are intended tobe included as falling within the broad scope and spirit of theforegoing written description, and the following claims, abstract of thedisclosure, and drawings.

We claim:
 1. A method comprising applying an adhesive to a firstsubstrate and a second substrate to secure said first substrate to saidsecond substrate, said adhesive extending in a plane on one side of aninterposer, said interposer also extending in said plane and contiguouswith said adhesive, said interposer comprising openings to enable theflow of said adhesive through said openings to form adhesive bond areason one of said substrates that substantially conform to said openings,said bond areas being adjacent to adhesive free areas, the other of saidsubstrate being substantially covered by said adhesive, so that saidadhesive bond areas produce regions of reduced adhesive strength to saidone substrate compared to the strength of the bond of said adhesive tosaid other substrate, where one of said substrates comprises a VTMelectronic device, and the other of said substrates comprises a heatspreader substrate.
 2. The method of claim 1 wherein said VTM electronicdevice comprises a heatable substrate and the other of said substratescomprises a heat spreader substrate.
 3. The method of claim 2 whereinsaid interposer comprises a heat conductive material.
 4. The method ofclaim 2 wherein said adhesive comprises a TIM.
 5. The method of claim 4wherein said openings are sized to produce said bond areas that allowseparation of said heat spreader from said VTM without damaging saidVTM.
 6. The method of claim 4 wherein said openings are sized to producesaid bond areas that allow separation of said heat spreader from saidVTM without damaging said VTM, where said VTM has a compressive limit ofabout 15 psi and tensile limit of about 7 psi.
 7. The method of claim 4wherein said TIM comprises a curable or a cured TIM.
 8. An article ofmanufacture comprising an adhesive on a first substrate and a secondsubstrate, said adhesive securing said first substrate to said secondsubstrate, said adhesive extending in a plane on one side of aninterposer, said interposer also extending in said plane and contiguouswith said adhesive, said interposer comprising openings through whichsaid adhesive flows and defines adhesive bond areas on one of saidsubstrates, said bond areas substantially conforming to said openings,said bond areas being adjacent to adhesive free areas, the other of saidsubstrate being substantially covered by said adhesive, so that saidbond areas comprise regions of reduced adhesive strength to said onesubstrate compared to the strength of the bond of said adhesive to saidother substrate, where one of said substrates comprises a VTM electronicdevice, and the other of said substrates comprises a heat spreadersubstrate.
 9. The article of manufacture of claim 8 wherein said VTMelectronic device comprises a heatable substrate and the other of saidsubstrates comprises a heat spreader substrate.
 10. The article ofmanufacture of claim 9 wherein said interposer comprises a heatconductive material.
 11. The article of manufacture of claim 9 whereinsaid adhesive comprises a TIM.
 12. The article of manufacture of claim11 wherein said openings are sized to produce said bond areas that allowseparation of said heat spreader from said VTM electronic device withoutdamaging said VTM.
 13. The article of manufacture of claim 11 whereinsaid openings are sized to produce said bond areas that allow separationof said heat spreader from said VTM without damaging said VTM, wheresaid VTM has a compressive limit of about 15 psi and tensile limit ofabout 7 psi.
 14. The article of manufacture of claim 11 wherein said TIMcomprises a curable or a cured TIM.